A passive optical network (PON) includes an optical line terminal (OLT) and multiple optical network terminals (ONTs) connected together using a passive optical splitter over a single fiber. The ONT transmits data at one wavelength (such as 1310 nm) and receives data at another wavelength (such as 1490 nm), and the OLT does the opposite. Optionally, a video channel may be transmitted between the OLT and ONT at yet another wavelength (such as 1550 nm). Because multiple ONTs may simultaneously transmit to one OLT, the data is typically time division multiplexed, wherein each ONT is assigned time slots at which it can transmit on the PON without colliding with other ONTs. Therefore, the ONT generally requires a burst mode transmitter that communicates with a burst mode receiver on the OLT.
The link performance of a PON system is dependent upon the characteristics of the output optical waveform from the terminals and is particularly important for the ONT because the ONT transmitter operates in burst mode. Standards, such as the G.984.2 standard, have been promulgated in order to assure interoperability among equipment manufacturers. The G.984.2 standard, among other things, defines standards for the output optical waveform.
Generally, there are two types of OLTs: RESET and RESETless. The RESET-based OLTs adjust the threshold between bursts, whereas the RESETless-based OLTs have a relatively low threshold. Therefore, RESETLESS-based OLTs generally require a high extinction ratio and fast rise and fall times in excess of the G.984.2 standard in order to achieve error free upstream burst mode communications from ONT to OLT.
Furthermore, PON optical transceivers employed at the OLT and the ONT have differing requirements. The ONT transceiver provides burst mode transmission and continuous mode reception, while the OLT transceiver is the opposite with continuous transmission and burst mode reception.
The transceiver performance varies with time and temperature. With both time and temperature the threshold current tends to rise and the slope efficiency decreases. As a result, calibration occurring at beginning-of-life may not hold until end-of-life.
Previous attempts to account for this variation focused on single open loop and dual open loop control systems. In these modes each module is calibrated over a temperature range to create look-up tables (LUT). A look-up table entry defines points typically ranging from −40° C. to +85° C. In dual open loop control system, a value for bias and modulation current is computed at each temperature. In single open loop mode, either modulation or bias current is set with a LUT and the other parameter is set via closed loop control. A temperature sensor senses the laser temperature and determines the correct bias and/or modulation current from the look up table. These values are loaded into a laser diode driver that controls the laser. The temperature compensation typically occurs in 3 degrees Celsius increments.
These methods, however, are cumbersome and require external test equipment, in addition to extensive calibration, accurate temperature measurement, and potentially extra components such as an EEPROM and a microcontroller. Furthermore, these methods fail to account for variations due to aging. The net effect of aging is increasing bias current for the same power, variation in threshold current, and decrease in slope efficiency, thereby degrading system performance over time. In cases where the better performance is required (such as RESETless OLTs), the degradation may be more pronounced.
Another method for controlling the laser diode is called dual closed loop control. In this method, the transmitted optical signal is detected and fed back to the laser driver. The feedback signal is used to control the power level of a digital “1” and a digital “0” independently. This maintains a constant output power and extinction ratio. This method also enables setting of high extinction ratios, which is beneficial for RESETless OLTs. It also compensates for both temperature and aging effects.
The dual closed loop control method when working in a PON network has several challenges. The dual closed loop control requires a lot of data to flow before the loop can converge. In particular, it needs a pattern of 5 ones and 5 zeroes occurring repeatedly to converge. For this to occur, successful communications must be established with the OLT. This process is called ranging.
A calibration procedure and manufacturing process are required to enable successful operation and manufacture of dual loop control transceivers in a PON network. The calibration must set up the transceivers initially as a function of temperature to guarantee ranging. Furthermore, it must program the dual loop control registers so that the output power and extinction ratio are maintained within specifications, and set up the receiver so that the sensitivity is within specifications across a temperature range.
Manufacturing of a dual loop control transceiver with discrete optics for an ONT has several advantages. The cost of extra components such as the printed wiring board (PWB), connectors, microcontroller, and EEPROM may be removed. The PON SOC (system-on-a-chip) can be used for control and PON memory that already exists on the board may be used for storage. The dual loop control method accounts for aging and temperature. However, a good calibration and manufacturing method is needed that results in lower costs due to simplified calibration, minimal testing, and improved manufacturability.
Therefore, there is a need for a system and a method for calibrating optical networking equipment. Also a process is required to streamline manufacturing of these transceivers with minimal testing.